I have an inline water heater under my kitchen sink. It creates 170 degree F hot water instantly and I love the fact that I can get a cup of tea anytime I want (like now as I’m writing this). The point of use tankless water heater is relatively small and will usually fit inside a sink cabinet or in a closet. They are typically dedicated use heaters meaning the unit serves one sink / faucet or one shower, etc.

An inline water heater are less expensive than whole house units and cost around a few hundred dollars for the unit (without installation).

Whole house units mean that they have higher gallons per minute flow rate capacity and can handle demand for more than one fixture at a time. For example a unit may handle two shower fixtures at one time or a dishwasher, kitchen sink and lavatory hot water faucet at one time. Why the different number of fixtures? Because different fixtures use different amounts of water. Some shower heads can use six times more water than a bathroom lavatory faucet.

So the size and number of whole house tankless water heaters you need will be largely driven by flow rate and that is determined by the number and types of fixtures you may have running at one time. And the worst culprits are shower heads. That’s why you may need more than one whole house tankless water heater hooked up in parallel to meet your hot water demands, especially for simultaneous shower usage.

Whole house units are much more expensive than point of use units and cost from several hundred to several thousand dollars (without installation).

In addition to the number and type of fixtures you want simultaneously served by the tankless water heater, you will also have to consider the temperature of your groundwater. And that is determined by where in the country you live.

In the chart above I have delineated what is generally accepted as the cool / warm dividing line of about 55°F groundwater. The colder the groundwater gets the less hot water can be produced by a unit for a given gallons per minute flow rate. This means a tankless water heater in Florida would have to be rated 33% to 50% larger in Michigan to serve the same number and types of fixtures.

Why? Well because the tankless unit heater has to warm the incoming cold water more in Michigan than Florida since the groundwater coming into the unit can be 30°F colder in Michigan (42°F) than Florida (72°F).

The three variables that have to be considered in sizing and selecting the unit include:

• The volume of water the unit is required to heat, measured as flow rate.
• The temperature of the cold water entering the unit
• The desired temperature of the hot water exiting the unit.

The difference between the the temperature of the hot water exiting the heater and the cold water entering the unit is called the temperature rise. If you want a shower up to 110°F and you live in south Florida with groundwater at 72°F, then you need a 38°F temperature rise (110-72=38).

A tankless water heater is sized by rating its temperature rise at a given GPM. So a unit could be rated at a 33°F Temperature Rise at 2.0 GPM. Based on manufacturer’s data, this same unit could also provide a 65°F Temperature Rise at 1.0 GPM.